It is known that the physicochemical properties of solids in particulate form are influenced by the size and shape of the particles. As particle size of solids diminishes in scale, there is an enhancement of properties, and often the inception of new properties.
New commercial products are becoming available which provide special advantage because of fine particle size. Zinc oxide is widely utilized as an ingredient in human health products. Superior results are now obtained by the use of submicron transparent zinc oxide powder.
Alkali metal bicarbonate is another commodity reagent which has found application in a broad variety of products such as laundry detergents, deodorizers, creams and lotions, dentifrices, antacids, buffers, fungicides, and the like.
There is evidence that fine particle size alkali metal bicarbonate can exhibit increased reactivity in comparison with coarse grain bicarbonate salts. In soda cracker production, finely divided sodium bicarbonate or potassium bicarbonate ingredient is more efficiently distributed and effectively reactive during the cracker dough preparation. The finished baked cracker is an improved product which has a substantially uniform texture, flavor and surface color, and a consistent pH throughout.
The inclusion of particulate alkali metal bicarbonate in an antiperspirant-deodorant cosmetic stick provides a product with improved deodorant properties. However, coarse or agglomerated particles of alkali metal bicarbonate have an undesirable tendency to settle in an antiperspirant-deodorant cosmetic stick matrix. The use of free-flowing ultrafine alkali metal bicarbonate as a deodorant ingredient in cosmetic stick and roll-on type personal care products is being investigated, since the ultrafine particles have less tendency to settle than coarse grain particles when dispersed in a liquid or semi-solid matrix.
A limiting factor has been the unavailability of alkali metal bicarbonate powder which is composed of free-flowing ultrafine particles that are not in an agglomerated state. Under ambient conditions of temperature and relative humidity, fine grain potassium bicarbonate powder undergoes caking in a short period of time and gradually converts into a hard mass.
The technical literature reports various means which have been developed for controlling the caking tendency in specific types of bulk inorganic powders.
U.S. Pat. No. 1,150,901 describes a means of stabilizing sodium bicarbonate powder by treating the powder with a saturated aqueous solution of di-sodium phosphate, and removing the water to provide sodium bicarbonate which is admixed with about 3% di-sodium phosphate as a stabilizing agent.
U.S. Pat. No. 1,869,235 describes a method of stabilizing hygroscopic ammonium bicarbonate powder by treating the powder with aqueous sodium chloride to cause a conversion of the ammonium bicarbonate on the particle surfaces to sodium bicarbonate and ammonium chloride, which form a protective coating on the particle surfaces.
U.S. Pat. No. 1,869,518 describes the production of free-flowing alkali metal iodide powder by admixing the powder with about 2% of an alkaline earth metal oxide. The metal oxide additive functions as a moisture scavenger and prevents the hygroscopic alkali metal iodide from absorbing water and suffering loss of free-flow properties.
U.S. Pat. No. 1,894,149 describes a process for stabilizing particulate ammonium bicarbonate which involves contacting the ammonium bicarbonate with ammonium chloride vapor to form a protective coating on the bicarbonate particles.
U.S. Pat. No. 1,907,076 describes a process for preparing an inorganic salt such as hygroscopic calcium chloride in a free-flowing form, which involves admixing the inorganic salt with an alkaline earth metal oxide additive such as magnesium oxide or calcium oxide. The oxide additive chemically combines with the calcium chloride water of crystallization to form a protective coating of alkaline earth metal hydroxide on the surface of the calcium chloride particles.
U.S. Pat. No. 2,218,031 describes a method for maintaining particulate sodium perborate tetrahydrate (NaBO.sub.3.4 H.sub.2 O) is a free-flowing state, which involves admixing the perborate with a small amount of magnesium oxide that has a low bulk density. It was found that magnesium hydroxide was ineffective as a free-flow aid in bulk sodium perborate tetrahydrate.
There is continuing interest in the development of methods for providing inorganic powders which remain in a free-flowing state, particularly when the powders normally are susceptible to agglomeration under ambient conditions of temperature and relative humidity.
Accordingly, it is an object of this invention to provide a process for preparing particulate potassium bicarbonate in a form which maintains its free-flowing properties when stored under ambient conditions of temperature and relative humidity.
It is another object of this invention to provide a free-flowing particulate potassium bicarbonate composition which has a protective coating physically-bonded on the surface of the potassium bicarbonate particles.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.